The following patent publications are believed to represent the current state of the art:
U.S. Pat. Nos. 3,395,353; 4,497,068; 4,857,760; 4,951,044; 5,241,160; 5,349,173; 5,357,091; 5,539,188; 5,640,003; 5,698,838; 5,801,372; 5,812,942; 5,815,020; 5,825,329; 5,831,257; 5,864,126; 5,874,725; 5,978,655; 6,021,951; 6,074,312; RE36769; 6,161,762; 6,202,927; 6,209,592; 6,234,902; 6,329,139; 6,375,082; 6,445,743; 6,507,130; 6,517,000; 6,533,178; 6,601,770; 6,686,847; 6,769,604; 6,883,103; 7,064,651; 7,075,434; 7,091,412; 7,162,302; 7,183,929; 7,207,488; 7,260,221; 7,280,970; 7,334,735 and 7,383,297.